The lac Operon

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The lac Operon
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Diauxic Growth Curve
Time in hours
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What was happening?
The cells use glucose first. When
they run out of glucose, they use lactose.
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Beta galactosidase is not expressed unless 1)
There is no glucose. 2) There is lactose.
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Two categories of transcription regulation

• Negative control repressor protein present which
  prevents transcription,inducer (usually a small
  molecule) is needed to allow initiation of
  transcription.
• Positive control activator protein induces
  transcription. No repressor must be overridden.

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Repressor Proteins

• Repressor proteins bind to an operator site
  within the promoter.
• Binding of a repressor prevents binding of RNA
  polymerase
• The repressor is an allosteric protein that can
  be activated by binding to a small molecule

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Repressor Proteins

• A repressor is a protein product made by one gene
  in the bacterium in order to control a second
  gene by turning it off when its product is not
  wanted.

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Activator Proteins

• Activator proteins bind to DNA sequences and then
  interact with RNA polymerase to enhance its
  binding.
• Activator proteins can be allosteric and bind to
  small molecules before they can bind to DNA.

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Activator Proteins
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Regulation of tryptophan synthesis
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The operon
A group of genes whose expression is coordinated
by an operator (F. Jacob et al.) 1960.
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The promoter
A genetic element necessary for the expression of
an operon (F. Jacob et al.) 1964.
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Lactose metabolism in E. coli.

• Enzymes necessary for lactose metabolism
• ß-galactosidase (cleaves lactose into glucose and
  galactose)
• lactose permease (transports lactose into the
  cell)
• transacetylase (function unknown)

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The lac operon

• lacZ (ß-galactosidase)
• lacY (lactose permease)
• lacA (transacetylase)

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Negative control of lac operon
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• Glucose levels are correlated to the
  concentration of cAMP via the enzyme that
  synthesizes cAMP, adenylyl cyclase.
• High glucose levels adenylyl cyclase
  low cAMP levels
• Low glucose levels adenylyl cyclase
  high cAMP levels